This invention relates to electronic ballasts for gas discharge lamps and, in particular, to an electronic ballast which can be dimmed by any external cause of reduced line voltage, including "brown-out" conditions imposed by electric utilities.
A gas discharge lamp, such as a fluorescent lamp, is a non-linear load to a power line, i.e. the current through the lamp is not directly proportional to the voltage across the lamp. Current through the lamp is zero until a minimum voltage is reached, then the lamp begins to conduct. Once the lamp conducts, the current will increase rapidly unless there is a ballast in series with the lamp to limit current.
A resistor can be used as a ballast but a resistor consumes power, thereby decreasing efficiency, measured in lumens per watt. A "magnetic" ballast is an inductor in series with the lamp and is more efficient than a resistor but is physically large and heavy. A large inductor is required because impedance is a function of frequency and power lines operate at low frequency (50-60 hz.)
An electronic ballast typically includes a rectifier for changing the alternating current (AC) from a power line to direct current (DC) and an inverter for changing the direct current to alternating current at high frequency, typically 25-60 khz. Since a frequency much higher than 50-60 hz. is used, the inductors for an electronic ballast can be much smaller than the inductors for a magnetic ballast.
Converting from alternating current to direct current is usually done with a full wave or bridge rectifier. A filter capacitor on the output of the rectifier stores energy for powering the inverter. The voltage on the capacitor is not constant but has a 120 hz "ripple" that is more or less pronounced depending on the size of the capacitor and the amount of current drawn from the capacitor.
Some ballasts include a boost circuit between the rectifier and the inverter. As used herein, a "boost" circuit is a circuit which increases the DC voltage, e.g. from approximately 180 volts (assuming a 120 volt line voltage) to 300 volts or more for operating a lamp, and which may provide power factor correction. "Power factor" is a figure of merit indicating whether or not a load in an AC circuit is equivalent to a pure resistance, i.e. indicating whether or not the voltage and current are sinusoidal and in phase. It is preferred that the load be the equivalent of a pure resistance (a power factor equal to one).
Because of the non-linear characteristics of a gas discharge lamp, dimming has long been a problem and many solutions have been proposed. Most dimmers include complicated circuitry and all dimmers require external access to the ballast, e.g. by wire running to a dedicated control, a knob on a control shaft extending from the ballast, or optical sensors. Until now, gas discharge lamps could not be controlled by dimmers intended for incandescent lamps, e.g. diodes, triacs, or variacs.
The simplest dimmer for an incandescent lamp is a diode in series with the lamp. The diode cuts off the positive or the negative portion of the A.C. waveform, thereby reducing the power applied to the lamp. Only two light levels are available with a diode, dim and bright. A triac dimmer uses switching circuitry to cut off a variable portion of the A.C. waveform to change the power delivered to a lamp. A variac is a variable transformer which reduces the voltage to a lamp for a range of light levels. A variac differs from a triac in that the output voltage from a variac is sinusoidal. Since many electronic ballasts require a sinusoidal line voltage in order to operate, a variac may seem a likely candidate for dimming a gas discharge lamp driven by an electronic ballast.
Most electronic ballasts sold today do not dim properly, if at all, in response to a reduced line voltage. A gas discharge lamp is essentially a constant voltage load on a ballast and, if lamp current decreases, the voltage across the lamp increases slightly. Consequently, most electronic ballasts stop working abruptly when the line voltage is reduced below a certain level. Thus, a variac cannot be used to dim gas discharge lamps driven by most electronic ballasts.
Some regulated electronic ballasts operate a lamp at constant power by drawing greatly increased current at reduced line voltages. Electrical utilities often control power distribution on a grid with "brown-outs" in which the line voltage is reduced by up to ten percent in some or all of the grid. Regulated power supplies, including ballasts, not only interfere with a utility's ability to control power consumption but make the problem worse by drawing even more current at reduced voltage in order to maintain constant power to a load; e.g. U.S. Pat. No. 4,220,896 (Paice). Unfortunately, the alternative has been to let gas discharge lamps flicker or go out. It is desired that an electronic ballast dim in response to reduced line voltage, thereby helping utilities to achieve their intended purpose with brown-outs.
There are many types of electronic ballasts and this invention relates to what are known as series resonant, parallel loaded ballasts. Such ballasts avoid the necessity of an output transformer by coupling a lamp in parallel with the capacitor of a series resonant inductor and capacitor. The inverter in such a ballast typically operates at a frequency slightly higher than the resonant frequency of the inductor and capacitor and dimming is achieved by raising the frequency of the inverter.
It is a characteristic of series resonant, parallel loaded ballasts of the prior art that the frequency of the inverter decreases as the line voltage decreases. For example, U.S. Pat. No. 4,677,345 (Nilssen) describes a series resonant, parallel loaded ballast in which the inverter includes a "half bridge," i.e. series connected switching transistors. A saturable reactor is connected in the base-emitter circuit of each transistor for switching the transistors at a frequency determined by the saturation time of the reactors. If the line voltage decreases, the reactors saturate more slowly and the frequency of the inverter decreases. As the frequency decreases, the series inductor presents less impedance and prevents lamp current from decreasing in proportion to line voltage. Thus, output power is relatively insensitive to line voltage.
In view of the foregoing, it is therefore an object of the invention to provide a self-dimming electronic ballast.
A further object of the invention is to provide an electronic ballast which reduces power to a gas discharge lamp in response to reduced line voltage to the ballast.
Another object of the invention is to provide an electronic ballast which operates a lamp at reduced power in response to a reduced input voltage, whether the reduced input voltage is from a reduced line voltage or from a dimmer connected between the ballast and an AC line.
A further object of the invention is to provide an electronic ballast for gas discharge lamps which can be on the same circuit as incandescent lamps and controlled by a single dimmer.
Another object of the invention is to provide an electronic ballast in which the frequency of the output current increases as the line voltage decreases.
A further object of the invention is to provide an electronic ballast having a series resonant, parallel loaded output driven by an inverter in which the frequency of the inverter is approximately inversely proportional to the line voltage.
Another object of the invention is to provide a dimmable electronic ballast costing the consumer little if anything more than a non-dimming electronic ballast.